1. Field of the Invention
The present invention relates to a receiver circuit for use in a relay station or in a mobile or portable terminal for a mobile communications system such as Personal Handyphone System (PHS).
2. Description of the Prior Art
In a mobile communications system such as Personal Handy-phone System, a base station, a relay station, and a portable terminal are interrelated as follows. The base station and the portable terminal communicate with each other by radio. The base station is installed, for example, at a pay-phone booth, and is linked to a land-based communications system (telephone network) across an ISDN line. In a situation where the portable terminal finds difficulty in establishing communication, as when it is used indoors, a relay station is installed, for example, by the side of a window of a building. In this case, the base station and the portable terminal communicate with each other through the relay station.
A conventional relay station operates as follows. A PHS signal received by the relay station through its antenna is fed to a radio-frequency circuit for amplification and other processing. Then, the received signal is demodulated by a demodulator into received data and a reproduction clock having a frequency of 384 kHz. Thereafter, the received data and the reproduction clock are fed to a succeeding processing circuit.
The demodulator requires a clock having a frequency higher than the reproduction clock for its operation. Accordingly, a temperature-compensated quartz-crystal oscillator (hereafter referred to as a "TCXO") is used. The TCXO outputs a clock having a frequency of 19.2 MHz. This clock is fed to the demodulator, and separately to an operation-clock generating circuit. The operation-clock generating circuit generates a bit clock having a frequency of 384 kHz through 1/50 frequency division of the 19.2 MHz clock and a frame synchronization clock having a frequency of 200 Hz through 1/96,000 frequency division of the 19.2 MHz clock.
The bit clock and the frame synchronization clock are fed to the processing circuit mentioned above. The processing circuit processes the received data by the use of the bit clock and the frame synchronization clock.
However, the base station and the relay station operate on the basis of independent clocks. In the relay station, the reproduction clock is produced from a signal transmitted from the base station, whereas the bit clock is produced through frequency division of the 19.2 MHz clock of the TCXO.
As a result, in the relay station, if the TCXO generates the clock at a deviated frequency, the reproduction clock does not synchronize exactly with the bit clock. This may result in data errors. Moreover, even if the reproduction clock and the bit clock have just the same frequency, they may have a phase difference. Such a phase difference remains uncorrected during signal processing, and makes the base station and the relay station distinguishable from each other in terms of their signals on the part of the portable terminal. This, however, is undesirable, because, ideally, the base station and the relay station should be treated with equity in terms of their signals on the part of the portable terminal.